442 'Journal of Comparative Neurology and Psychology. 



for example, by mechanical stimulus. The conventional negative 

 response of turning away from a localized stimulus is brought 

 about by contraction of the muscles of the unstimulated side. 

 Corethrahas as definite an avoiding reaction as any, but gets away 

 by contracting the stimulated side, even in this way bringing its body 

 at first into closer contact with the stimulating object. It is 

 evident that it is not the mode of orientation which is uniform in 

 different animals, but the result. The recognition of unconformity 

 in the process of orientation in different animals emphasizes the 

 self-regulative capacity of animals, and the difference between 

 their movements and those of passive machines. In any case the 

 most direct mode of orientation which is consistent with the ani- 

 mal's own system of movements is followed. Hence it is proper 

 to speak of movements as directed, remembering that activity, not 

 passivity, is implied in the process. 



APPLICATION OF THE TRIAL AND ERROR THEORY TO THE 



REACTIONS. 



It is frequently maintained or implied that reactions to external 

 stimuli lie at the foundation of behavior. But not all or even the 

 larger part of an animal's movements are referable to external 

 stimuli. In the case of Corethra the possibility of distinguishing 

 the different movements with reference to their exciting cause is 

 favored by their discontinuity. A significant addition to recog- 

 nized forms of reaction among the lower animals are the trial 

 movements emphasized by Jennings and others. Such move- 

 ments make up by far the larger part of an animal's behavior, as 

 is recognizable in a roughly quantitative way in Corethra, since 

 the separate movements of the larva stand out distinctly and give 

 opportunity to relate them to their antecedent causes, in a way 

 which is perhaps more obvious than in an ordinary, continuously 

 moving, restless water insect. In Corethra, while external, local- 

 ized stimuli give rise to oriented movements, internal or unlocal- 

 ized stimuli give rise to movements indefinite in direction but 

 adaptive in bringing the organism into as wide contact with the 

 environment as possible. As was pointed out above, wider range 

 of vision is secured in these larvae by the asymmetrical character 

 of their movements, which causes them to face successively in 



